The present invention generally relates to implantable prostheses and the like and to methods for making or treating same in order to substantially prevent cracking or crazing thereof when they are implanted or otherwise subjected to degradation conditions. A medical prosthesis or the like according to this invention includes a polycarbonate urethane polymeric surface which will not crack or degrade when subjected to implantation for substantial time periods during which other types of polyurethane surfaces would crack or degrade.
Several biocompatible materials which are quite suitable for use in making implantable medical devices that may be broadly characterized as implantable prostheses exhibit properties that are sought after in such devices, including one or more of exceptional biocompatibility, extrudability, moldability, good fiber forming properties, tensile strength, elasticity, durability and the like. However, some of these otherwise highly desirable materials exhibit a serious deficiency when implanted within the human body or otherwise subjected to harsh environments, such deficiency typically being manifested by the development of strength-reducing and unsightly cracks. For example, surface fissuring or cracking occurs after substantial exposure, which may be on the order of one month or more or shorter time periods depending upon the materials and the implant conditions and exposure to body fluids and cells such as are encountered during in vivo implantation and use. Many implantable prostheses are intended to be permanent in nature and should not develop any substantial degradation or cracking during years of implantation.
Several theories have been promulgated in attempting to define the cause of this cracking phenomenon. Proposed mechanisms include oxidative degradation, hydrolytic instability, enzymatic destruction, thermal and mechanical failure, immunochemical mechanisms, inhibition of lipids and combinations of the above. Prior attempts to control surface fissuring or cracking upon implantation or the like include incorporating antioxidants within a biocompatible polymer and subjecting the biocompatible polymer to various different annealing conditions, typically including attempting to remove stresses within the polymer by application of various heating and cooling conditions. Attempts such as these have been largely unsuccessful.
Other treatment approaches have been utilized, or attempted, to increase the structural stability of especially desirable materials. Included in the biocompatible materials which are desirable from many points of view but which exhibit a marked tendency to crack or degrade over time are the polyurethane materials and other biocompatible polymers that are of an elastomeric nature. It is particularly advantageous to use these types of materials for making products in respect of which compliance and/or flexibility, high tensile strength and excellent fatigue life can be desirable features. One basic approach which has been taken heretofore in order to render these materials more suitable for implantation and other applications where material degradation can develop has been to treat the material with so-called crack preventatives. Exemplary approaches in this regard are found in U.S. Pat. Nos. 4,769,030, No. 4,851,009 and U.S. Pat. No. 4,882,148, the subject matter thereof being incorporated by reference hereinto. Treatments, of course, require additional procedures and components, thereby somewhat complicating manufacturing procedures, and it would be advantageous if the material out of which the product is made would itself have the desired properties. It is also advantageous for the material to be compatible with other materials that are commonly used in the medical fields such as with adhesives, surface coatings and the like.
An especially difficult problem in this regard is experienced when attempting to form prostheses with procedures including the extrusion or spinning of polymeric fibers, such as are involved in winding fiber-forming polymers into porous vascular grafts or similar products, for example as described in U.S. Pat. No. 4,475,972, the subject matter thereof being incorporated by reference hereinto. Such vascular grafts or the like include a plurality of strands that are of a somewhat fine diameter size such that, when cracking develops after implantation, this cracking often manifests itself in the form of complete severance of various strands of the device. Such strand severance cannot be tolerated to any substantial degree and still hope to provide a device that can be successfully implanted or installed on a generally permanent basis whereby the device remains viable for a number of years.
Numerous polymeric structures such as vascular grafts made from spun fibers appear to perform very satisfactorily insofar as their viability when subjected to physical stress conditions is concerned, for example conditions which approximate those experienced during and after implantation, including stresses imparted by sutures, other fastening members and the like. For example, certain polyurethane fibers, when subjected to constant stress under in vitro conditions, such as in saline solution at body temperatures, do not demonstrate cracking that is evident when substantially the same polyurethane spun fibers are subjected to in vivo conditions. Accordingly, while many materials, such as certain various polyurethanes, polypropylenes, polymethylmethacrylates and the like, may appear to provide superior medical devices or prostheses when subjected to stresses under in vitro conditions, they are found to be less than satisfactory when subjected to substantially the same types of stresses but under in vivo conditions.
There is accordingly a need for a material which will not experience surface fissuring or cracking under implanted or in vivo conditions and which is otherwise desirable and advantageous as a material for medical devices or prostheses that must successfully delay, if not eliminate, the cracking phenomenon even after implantation for months and years, in many cases a substantial number of years. In addition, other products which are not necessarily intended for medical use can benefit from their being made of such a non-cracking material. Products in this regard could include those subjected to harsh environmental conditions such as weathering and the like. Exemplary medical devices or prostheses for which such a non-cracking material would be especially advantageous include vascular grafts, compliant sutures, breast implants, heart leaflet valves, pacemaker lead insulators, intraocular lens loops or haptics, diaphragms for artificial hearts, tubing for infusion pumps, artificial ligaments, artificial skin, drug eluting matrices, lattices for cell seeding and artificial organs, and the like. Examples of non-medical applications of these urethanes include urethane for roofing insulators, sewer gaskets, industrial tubing and the like.
In summary, the present invention achieves these types of objectives by providing a polycarbonate urethane polymer as the material out of which crack-resistant products are made. The polycarbonate urethane polymer is exceptionally crack-resistant, even under in vivo conditions. The polymeric backbone has recurring urethane and/or urea groups, and the polymer is a reaction product of at least a polycarbonate glycol having terminal hydroxyl groups and a diisocyanate having terminal isocyanate groups. A chain extender having terminal hydroxyl or amine groups may or may not be added. It is especially preferred that the resultant hardness of the polycarbonate urethane be at least as hard as about Shore 70A, preferably between about Shore 80A and Shore 75D.
It is accordingly a general object of the present invention to provide improved crack-resistant devices and products.
Another object of this invention is to provide a polymeric material and products made therefrom which are particularly resistant to degradation, even under in vivo conditions.
Another object of the present invention is to provide an improved polyurethane type of material which can be spun through a spinnerette or extruded through and/or into suitable molding devices into products which exhibit superior crack-resistant properties.
Another object of this invention is to provide improved implantable devices and/or prostheses which exhibit an exceptional ability to prevent the formation of cracks and strand severances upon implantation for substantial time periods such as those needed for generally permanent implantation procedures.
Another object of the present invention is to provide an improved vascular graft and the like that is made from spun fibers of polycarbonate urethane polymer and that exhibits exceptional stability with respect to crack formation and strand severance development under in vivo conditions.
Another object of the present invention is to provide an improved extruded device or product that is unusually crack-resistant, even when subjected to harsh environmental conditions.
Another object of the present invention is to provide improved cast polymer products including a polycarbonate urethane polymer which exhibits exceptional crack-resistance.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.